Wire bonding is a common procedure used in semiconductor packaging whereby electrical wire connections are made between electrical contact pads of different electronic components, or of different contacts pads on an electronic component. For example, wire connections are commonly made between contact pads of an integrated circuit (“IC”) chip and a lead frame carrier on which the IC chip is attached before the IC chip is molded during assembly.
A capillary is often used to apply bonding energy to weld bonding wire, typically made from gold, aluminum or copper, to the contact pads. The capillary is also used to feed wire to the contact pad and break the wire from the bond on the contact pad once a wire connection is made. Bonding energy, such as ultrasonic force, power and/or thermal energy is applied at a tip of the capillary. Therefore, it is very important for the capillary to be fastened securely so as to ensure that the wire bonds are formed properly and consistently according to set parameters.
Conventionally, a horn of an ultrasonic transducer is formed with a slit at its tip. The capillary is mounted in a recess in the slit. Threaded screw holes are made in the slit to receive a screw, which is screwed into the holes and tightened in order to close the slit and provided a clamping force to grip the capillary securely. However, it has been found that there are many disadvantages associated with the use of a screw to secure the capillary. One disadvantage is that since wire bonding is performed repeatedly at high frequency under stressful conditions, over time, the screw threads on the screw and screw holes become worn and loosen the screw. The problem is that wire bonding requirements are so precise that any loosening of the capillary mount may adversely affect the performance of the wire bonding tool. Another undesirable effect is the possible bending vibration caused by the screw.
One method of avoiding the aforesaid disadvantage is to utilize the body of the horn itself to grip the capillary without reliance on a foreign object to secure the capillary. This approach is disclosed in U.S. Pat. No. 6,422,448 entitled “Ultrasonic Horn for a Bonding Apparatus”. An ultrasonic horn is provided with a capillary attachment hole formed so as to be smaller than the capillary and a jig insertion hole that communicates with the capillary attachment hole. A jig is insertable into the jig insertion hole to widen the capillary attachment hole. Therefore, a cross-sectional area of the capillary attachment hole is increase, so that the capillary can be received in the capillary attachment hole. The capillary is fastened in place by the elastic force that is generated by the horn material when the jig is removed from the jig insertion hole and the cross-sectional area of the capillary attachment hole is thereby reduced.
In typical ultrasonic transducers, the ultrasonic drivers are arranged such that ultrasonic oscillation is generated down the horn in a longitudinal direction along the length of the horn. A disadvantage of the gripping force introduced by the aforesaid prior art is that the elastic force is exerted on sides of the capillary that are transverse to the longitudinal oscillation axis of the horn. By gripping the capillary transversely to this longitudinal direction, the gripping force is less effective, and there is a greater risk of movement of the capillary in the longitudinal direction relative to the horn if the elastic fastening force is not sufficiently large to prevent this.